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Common Types of Building Defects
- Foundation issues
- Cantilever structures
- Water seepage
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Causes of soil movement
- Moisture –volume of soil changes with moisture contents
- Pressure –soil consolidate under pressure/loading
- Vibration –traffic, piling, earthquake, etc.
- Faulty foundation design / construction
- Settlement of reclaimed / landfill site
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Settlement and reclaimed land - All reclaimed land will settle, Allow for … years generally for settlement, the faster the settlement, ……; Speed and amount depends on…
- 5-7; the faster the construction can begin
- fill materials (e.g. sand) and the seabed material
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TKO building settlement – cause
- reclaimed land
- Strategic sewage discharge program-sewage tunnel built under TKO; inflow of ground water into the tunnel → ground water level drops → further settlement
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TKO building settlement –consequence
- superstructure generally not affected
- joints between building and roads may crack appearance
- underground services e.g. burst pipes
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Type of piles and settlement
- Friction piles –more problematic: likely to sink with soil together
- end-bearing piles: loads transferred to bedrock → building itself is stable (doesn’t settle)
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TKO building settlement –remedies
- seal up the cracks
- use flexible joints
- On-site testing to ensure quality of piles (e.g. ultrasonic test)
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Cantilevered Structures – reason for collapse
- Tension on top → main reinforcement on top; Cracks more likely on surface under tension
- Main reinforcement on top may be lower than designed, eg. Workers walking on top during construction strength weakened
- The top may often get wet (cracks on slab surface)corrosion
- Excessive loading, eg. Wind load, extra loading
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Cantilevered Structures-remedies for collapse
- Avoid premature removal of props supporting the formwork (min. 28 days)
- Precast balcony –better quality control
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Common Mechanisms of Water Penetration
- Gravity (Gravity force pulls water down the face of a wall and into possible openings)
- Wind-driven
- Capillary action and surface tension (Water can flow against gravity and in narrow spaces)
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Buildings Elements Vulnerable to Water Seepage
- Flat Roofs
- Windows
- Floor Slabs
- Partition Walls
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Buildings Elements Vulnerable to Water Seepage- Flat Roofs-why
- Weather attack: rain, heat, wind
- Building services (大廈管理)
- Misuse/Illegal Structure
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Problems with flat roofs
- Poor Roof Maintenance
- Ponding 1: Improper Grade of Slab Surfaces(deflected beam) + insufficient fall + drain at high point
- Ponding 2: Blockage of drainage outlets; use dome-shaped drainage outlets
- Additional works on top of roof; Shock absorber for ac
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Flat roof- Common materials involved
- Waterproof Membranes(sheet form+liquid form)
- Asphalt
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Defects of Asphalt
- Surface crazing (minor cracks)-Due to sun exposure
- Fractures (Deep cracks throughout the asphalt thickness)-Due to excessive bending
- Blistering(Asphalt softened under solar heat)- Due to expansion of entrapped water
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Good Flat Roof Design
- Adequate movement joints
- Avoid too many openings
- Avoid overloading
- Allow sufficient fall (1:40 recommended)
- Install sufficient floor drains, use dome-shaped drainage outlets
- Shock absorber for ac
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Buildings Elements Vulnerable to Water Seepage- Windows-why
- Gap too wide for grouting
- Waterproof coating applied to concrete surface (before grouting)
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Buildings Elements Vulnerable to Water Seepage- Windows-remedies
- Projections over window
- Drip groove under projections to break surface tension
- Set-back of Window
- Non-shrinkage cement grout to fill up the gap
- Waterproof coating on grout surface
- Series of water tests conducted before and after grouting
- Applying sealant around window
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Weakness of aluminum windows and regulations
- corrosion of screws/rivets window sash falls apart
- Mandatory Window Inspection Scheme –inspection every 5 years for buildings 10 years or older
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